One of the requirements for trade dress in the US is that the look has been exclusively in use for a while.
That is an oversimplification.
In the US, a propduct's Trade Dress is covered by the Lanham Act. After five years trade Dress becomes "incontestible" - ie. another company cannot
challenge the right of (for example) Coca-Cola to the distinctive shaped bottle. But there is no reason that a company cannot assert Trade Dress rights and protections from day one. (And nowadays, most companies do just that.)
In general, its best to keep the following in mind when considering a company's rights to Trade Dress.
1) The Trade Dress characteristic cannot be
functional. In other words, Apple cannot claim that simply having icons on a capacitive touchscreen is an element of Trade Dress, because the icons (and touchscreen) provide a function. (The capacitive touchscreen and method of interacting with it may be covered by
patents, but that is another discussion.)
However, Apple certainly CAN claim that the overal
appearance of the device - and specifically the shiny metallic bezel that surrounds the touchscreen, ARE elements of Trade Dress. Samsung certainly could have made a Galaxy Tab that
functioned without the metallic bezel. They didn't - they chose to make a product that looked, to a very great degree, just like the iPad.
2) The second element of Trade Dress is that the elements one claims must be
distinctive.
Thats where people posting pictures of Microsoft Tablets and the like are missing the point. Yes - these items may meet the definition of Tablets. But none of them have the specific design elements Apple is claiming as Trade Dress.
Just as plenty of people had sold beverages in glass bottles for (literally) hundreds of years before Coca-Cola came along with their product. Coca Cola never claimed to have invented the idea of serving drinks in bottles. What it claimed (and now
incontestibly owns) is the right to sell them in distinctive hour-glass shaped bottles.
What Apple is doing, under their trade Dress registrations, is claiming the right to sell capacitive touch-screen computing tablets with metallic bezels (this is somewhat of an oversimplification.) But I think a fuller understanding of the law and how it is applied, both in Europe and here in the US, tends to support their argument.